Adaptive solar powered system

ABSTRACT

An apparatus for powering a load from solar energy comprises a DC/DC converter, battery terminals coupled to an output of the DC/DC converter and coupleable to an energy storage device, and solar panel terminals coupled to an input of the DC/DC converter and coupleable to a solar panel. Logic is coupled to the converter to control its conversion rate. A detector is coupled to the converter and detects its power output. The logic is operable to adjust the conversion rate until the solar panel operates at the smaller of a maximum power point of the solar panel and a power point of the solar panel that results in a maximum desired power.

FIELD OF THE INVENTION

This invention relates to solar power systems and in particular to powermanagement subsystems for solar power systems.

BACKGROUND OF THE INVENTION

Solar powered systems provide benefits such as reduced installationcost, the ability to be installed in areas where no other source ofenergy is available, etc. Typically, a solar powered system will consistof a solar panel, battery, load (such as a light emitting diode) andassociated circuitry. The associated circuitry controls charging of thebattery, load operating times and load operating levels.

Sizing of the solar panel or panels, battery or batteries for a giveninstallation requires many inputs for optimal cost for a givenperformance requirement. For instance, the latitude and weatherconditions in a given installation location significantly affect theamount of power available to the system. In addition, if the location ofinstallation can be shaded by buildings, trees, etc., it may bedesirable to reduce the load levels, load operating times, increase thesize of the solar panel, increase the size of the battery, etc. in orderfor continued operation of the device.

Solar powered systems may be used in many different applications. Theseapplications include marine beacons, aviation beacons, roadway signals,transit shelter lighting systems, roadway signs, etc. Some applicationshave known operating times which may be taken into account during thedesign of the system. Other applications have operating times that aredetermined by the user of the system. When the operating times aredetermined by the user, design of the system becomes more difficultbecause the amount of energy used by the system over a given time periodmay be unknown at the time of design or installation of the system.

A solar power lighting system may have to be designed to provideadequate energy to the lights during the winter when the amount of solarinsolation may be reduced. This may be more of a consideration forlatitudes farther from the equator as day length is reduced by a greateramount during the winter than at latitudes closer to the equator.Typically a combination of sizing the solar panel to provide enoughenergy in the winter months and sizing the battery to carry adequateenergy through the shortest days of the year is used to ensure adequatelighting. For lighting systems designed to provide light primarilyduring the night, the effect of shorter days with less solar insolationis compounded by the fact that the nights are longer in the winter.Thus, the lighting system requires more energy. The solar panels andbatteries are often sized for sustained winter operation; thereforeduring the summer months, an excess amount of energy is available fromthe solar panels and/or batteries which typically cannot be utilized.

In some solar powered lighting applications, it may be desirable tocontrol the lighting based on the time of day. Since solar poweredlighting is often installed in areas where access to communicationsinfrastructure is limited, maintaining the time of day usingconventional means such as through wired or wireless communications maybe difficult.

SUMMARY OF THE INVENTION

The invention comprises elements of a solar power system and methods ofoperating the system. The system may be used for powering loads such aslight emitting diodes (LEDs), audio systems, communications systems,etc. The system is capable of making maximum use of available solarpower while continuing operation when available solar power is reduced.In an alternative implementation, the system is capable of operationwith smaller componentry when traditionally there would be too muchsolar insolation and system components would be damaged. The system maybe capable of determining the time of day by detecting when thebeginning and end of a day or night occurs. An alternativeimplementation of the system may be coupleable to an electric grid.

The solar powered system may come in various versions. These versionsmay include solar powered crosswalk beacons, solar powered school zonewarning beacons, solar powered traffic warning signals, solar poweredbus shelters, solar powered advertising panels, solar powered streetname signs, solar powered audio warning signals, or various otherversions where powering a load from solar power is advantageous.

In one aspect, the invention comprises an apparatus for powering a loadfrom solar energy comprising a DC/DC converter, battery terminalscoupled to an output of the converter and coupleable to an energystorage device, solar panel terminals coupled to an input of theconverter and coupleable to a solar panels, and logic coupled to theconverter to control the conversion rate of the converter. A detector iscoupled to the converter to detect the power output of the converter.Logic is operable to adjust the conversion rate until the solar paneloperates at the smaller of a maximum power point of the solar panel anda power point of the solar panel that results in a maximum desiredpower.

In another aspect, the invention comprises apparatus for powering a loadfrom solar energy comprising a DC/DC converter, battery terminalscoupled to the converter and coupleable to a battery and solar panelterminals coupled to the converter and coupleable to a solar panel.First logic is provided for controlling a conversion rate of theconverter to operate near the maximum power point of the solar panel. Adetector detects an energy output of the converter and second logic iscoupled with the detector to adjust the energy consumption of the loadover a period of time as a function of the energy available from thesolar panel over that period of time.

In yet another aspect, the invention comprises a method of determining atime to deliver an amount of energy to a load in a solar powered systemcomprising monitoring an output of a solar panel to determine a start ofday, monitoring the output of the solar panel to determine a start ofnight, rejecting an effect in said determination of said start of daycaused by artificial light, using the beginning of the night and thebeginning of the day to compute the current time of day and deliveringsaid amount of energy to the load based on the current time of day.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described by reference to following detaileddescription and to the drawings thereof in which:

FIG. 1 is a diagram of the system of an example of the invention;

FIG. 2 is a schematic of the componentry implementing an example of theinvention;

FIG. 3 is a set of solar panel current-voltage and power voltage curvesthat may be applied with an example of the invention;

FIG. 4 is a flowchart for the a portion of the operation of a processoraccording to an example of the invention;

FIG. 5 is a set of solar panel power-voltage plots with or without diodebypassing that may be applied with an example of the invention;

FIG. 6 is a flow diagram depicting a portion of the operation of saidprocessor for the assessment of the approximate time of day; and

FIG. 7 is a diagram of an alternate system of an example of theinvention.

DETAILED DESCRIPTION

FIG. 1 is a diagram of solar powered system 100. The solar poweredsystem comprises an energy management system (EMS) 110. The EMS 110 iscoupled to a solar panel 120 for receipt of energy from the sun. The EMS110 is coupled to a battery 150 for storage of energy. Alternatively,another energy storage device such as a supercapacitor, fuel cell, etc.may be used. The EMS 110 is coupled to a load 140 (such as an LED orLEDs) to which the EMS may provide power. Auxiliary circuitry 130 mayuse power from the EMS 110 or provide or receive other signals from theenergy management system 110. Auxiliary circuitry may comprise forexample a pedestrian pushbutton, motion detector, etc.

FIG. 2 is a schematic diagram of an example implementation of the EMS110. In this figure, standard support circuitry known to be necessary inthe field has been omitted for clarity. A processor 200 controls theoperation of the EMS 110. The processor may comprise a microcontroller,microprocessor or any applicable logic capable of carrying out thefunctions described herein. The processor 200 receives a signalindicative of the voltage of solar panel 120 through photovoltaicvoltage sensing circuitry 210. This may for example enter an A/D inputof the processor 200. This voltage may be the open circuit voltage ofthe solar panel 120 when switch 220 is open or the operating voltage ofthe solar panel 120 when switch 220 is closed. DC/DC converter 201receives power from the solar panel 120 and provides it to the battery(or batteries) 150 and/or the load 235. The processor 200 can controlthe application of power to the load 235 using switch/control circuitry202. Switch/control circuitry 202 may be a simple switch or more complexcircuitry capable of controlling the amount of voltage, current or powerdelivered to the load 235.

DC/DC converter output current sensing circuitry 280 detects the amountof current flowing from the DC/DC converter 201 and provides a signalindicative thereof to processor 200. Load current sensing circuitry 205detects the amount of current flowing to the load 235 and provides asignal indicative thereof to the processor 200. Battery voltage sensingcircuitry 290 detects the battery 150 voltage and provides a signalindicative thereof to the processor 200.

The DC/DC converter 201 receives power from the solar panel (or panels)120 and provides this power to supply load 235 and/or charge battery150. The DC/DC converter is disposed in H-bridge configuration.Alternatively, any other applicable DC/DC converter topology may beused. The DC/DC converter may operate in three different modes. Thefirst mode (buck) may used when the solar panel 120 voltage is higherthan the battery 150 voltage. The second mode (boost) may be used whenthe solar panel 120 voltage is lower than the battery 150 voltage. Thethird mode (buck/boost) may be used when the solar panel 120 voltage ishigher or lower than the battery 150 voltage.

In buck mode, switch 270 is always on, while switch 225 is always off.Switch 230 is controlled via a Pulse-Width Modulated (PWM) drive signalfrom processor 200. The duty cycle of the PWM signal of switch 230determines the ratio of the input and output voltage of the DC/DCconverter 201: Vout=Vin*d. Switch 240 can optionally be used to providesynchronous rectification by turning on when Switch 230 is off and visaversa. Synchronous rectification improves the efficiency of theconverter by reducing the voltage drop across schottky diode 250 duringthe time when switch 230 is not conducting.

In boost mode, switch 230 is always on and switch 240 is always off. Inthis configuration, switch 270 is controlled via a PWM drive signal fromprocessor 200, and switch 225 provides the optional synchronousrectification by turning on when switch 270 is off and visa-versa. Thisimproves efficiency by reducing the voltage drop across schottky diode215. Again, the duty cycle of the PWM signal determines the ratio of theinput and output voltages: Vout=Vin/(1−d).

In buck/boost mode, all four switches are driven simultaneously, inpairs. Switch 230 and switch 225 are driven in phase with each other,and switch 240 and switch 270 are driven in phase with each other. Whenthe 230/225 pair is turned on, the 240/270 pair is turned off. The dutycycle of the 230/225 pair determines the ratio of input and outputvoltages, which can vary over a wide range. This mode of operation maybe less efficient than the pure Buck, or pure Boost modes, but isoperable whether the output voltage is higher or lower than the inputvoltage.

Since the output voltage of the DC/DC converter 201 is the battery 150terminal voltage, which changes relatively slowly, the DC/DC convertercan be used to control the input voltage which is the solar panel 120voltage. By varying the duty cycle appropriately (depending on the modeof operation), the solar panel 120 can be adjusted until the powerdelivered to the battery 150 reaches any desired point on the solarpanel voltage/current characteristic.

The switches 220, 225, 230, 240, 270 and 202 may be MOSFETs configuredwith appropriate drive circuitry or another type of electronic switchappropriate for a given implementation.

FIG. 3 depicts characteristic curves for a solar panel 120.Characteristic panel current-voltage (IV) curves 300 show the currentoutput of the solar panel 120 for a given voltage. Summer IV curve 320shows what the IV curve of solar panel 120 may look like when availablepower is high during a summer day. Winter IV curve 330 shows what the IVcurve of solar panel 120 may look like when available power is lowerduring a winter day, the solar panel 120 is in the shade, etc.

Characteristic panel power-voltage (PV) curves 310 show the power outputavailable from solar panel 120 for a given voltage. Summer PV curve 370shows what the PV curve of solar panel 120 may look like when availablepower is high during a summer day. Winter PV curve 380 shows what the PVcurve of solar panel 120 may look like when available power is lowerduring a winter day, the solar panel 120 is in the shade, etc.

The maximum power point in summer is represented by point 340 a and 340b. The maximum power point in winter is represented by point 350 a and350 b. The processor 200 can adjust the DC/DC converter 201 to operateat or near these maximum power points.

Alternatively, it may be desirable to not operate the solar panel 120 atits maximum power point during certain times of the year. It may bedesirable to operate the solar panel 120 at points 360 a and 360b in thesummer due to the fact that less current will be flowing through theDC/DC converter 201 and therefore the current carrying capacity ofswitches 220, 225, 230, 240, inductor 260, schottky diodes 215 and 250,wiring, circuit board traces, etc. may be reduced with a correspondingreduction in the size and cost of these devices. This mode of operationis desirable because it is often necessary to size the solar panel 120and battery 150 to provide enough energy to operate the load duringoperation of the system in the winter. Therefore, the solar panel 120may have excess output during the summer and the battery 150 may nothave adequate capacity to store the excess energy the solar panel 120generates. This may be especially true the farther in latitude thesystem is installed from the equator and/or in geographic locationswhere cloudy weather is more prevalent at certain times of the year.

FIG. 4 is a flowchart of the operation of processor 200 to control DC/DCconverter 201. First, the processor 200 determines which mode to operatethe DC/DC converter in (block 400). This may be done in several wayssuch as measuring the open circuit voltage of the panel and estimatingwhether the solar panel 120 voltage of the desired power point will beabove or below the battery 150 voltage. Alternatively, the buck/boostmode may be used initially until the operating voltage of the solarpanel 120 stabilizes at a voltage above or below the battery voltage.

Next, the processor 200 sets the PWM duty cycle to the switchesappropriate for the selected mode to a minimum (block 410). This may befor instance a duty cycle of 1/64 for a 100kHz PWM signal. This makesthe solar panel 120 operate very near to its open circuit voltage. Thecurrent flowing from the DC/DC converter 201 is monitored (block 420).The current flowing is compared to the maximum current recorded duringthe current operating loop (block 430). If the present current is notthe maximum recorded during this operating loop, the duty cycle isincreased (block 450). If the present current is the maximum recordedduring this operating loop, the present current is compared to themaximum current allowed due to the circuit design (block 440). If thepresent current is greater than the maximum current allowed, executioncontinues at block 478 (described below). If the present current is notgreater than the maximum current allowed, the duty cycle and current ofthe present setting are recorded as the new maximum and executioncontinues at block 450. At block 470, the duty cycle is increased andexecution continues at block 420 until the maximum duty cycle has beentried. Alternatively other techniques such as looking at the rate ofchange of current may be used to find the maximum.

Once the maximum duty cycle has been tried, if the battery is at or nearfull charge (block 478), the DC/DC converter is set to a duty cycleresulting in a low current output to maintain the battery charge (block476). A record of the potential output power from the DC/DC converter isstored (block 477). This low current may be slightly higher than thepresent current being drawn by the load.

If the battery is not at or near full charge, the processor beginsoutputting the duty cycle of the detected maximum power point or maximumcurrent point (block 480). A record of the current output power from theDC/DC converter is stored (block 490).

The processor then operates the PWM outputs at the present setting for agiven period of time (block 495). This period of time may be for example5 minutes. During this time, the processor may monitor the currentflowing from the DC/DC converter 201 and restart the process if thecurrent changes by a significant amount. Using the record of the outputpower, the processor 200 then accumulates the amount of energy outputfrom the DC/DC converter (block 485). If 24 hours have not passed,execution continues at block 400.

If 24 hours have passed, the processor has then accumulated anindication of how much energy was generated, could have been generated,or a combination thereof from the solar panel 120 through the DC/DCconverter 201 over the previous day (block 465). Power records fromblock 490 are indicative of power actually generated whereas powerrecords from block 477 are indicative of power potentially generatedthat could not be stored due to the battery being near full charge. Thisinformation may then be used to set the allowable amount of energy to beused over the next day (block 455). This may be calculated such that theamount of energy to be used is a large fraction of that generated (forexample 90%) to take into account inefficiencies in the system andoperate the system in an energy balance so that the battery is notdepleted over the long term. It will be appreciated that using 24 hoursas a fundamental period of time in the above discussion is arbitrary andany appropriate period of time may be used depending on the application.When there is no solar insolation for an extended period of time, theprocessor 200 may enter a sleep mode by turning off switches 202, 225,230, 240, 270. This may allow the load to be operated for very shortperiods of time, during critical times, not at all, etc. while reducingthe extraction of energy from the battery 150 to the lowest levelpossible. Once solar insolation is detected again, the processor 200 mayexit this low power mode. This sleep mode may be used when a deviceincorporating the invention is in a shipping container, in storage,etc., with the battery 150 connected, but no solar insolation available.Thus, the device is ready to operate as soon as solar insolation isdetected, but it the device may be left without solar insolation for arelatively long period of time before the battery is completelydischarged and potentially damaged.

The amount of energy to use over the next day may be computed usingvarious methods using the amount of energy available over a preceedingnumber of days. For example:${Etouse} = {\sum( {{\frac{1}{2}E_{today}} + {\frac{1}{4}E_{yesterday}} + {\frac{1}{8}E_{{two\_ days}{\_ ago}}}} )}$

-   -   where Etouse is the amount of energy to use today,        -   E_(n) is the amount of energy available on day n.

The number of days to include in the calculation and the fraction toattribute to each day may be determined by the consistency of insolationat the installation location or any other appropriate technique. Ingeneral, any adaptive, averaging or smoothing function may be utilizedto produce a similar result.

The amount of energy to be used over a day can be adjusted using one ormore of several techniques including:

-   -   Reducing or increasing the brightness of a light such a an LED        by increasing or decreasing the amount of current flowing        through the LED or by pulsing the current through the LED for an        reduced or increased amount of time at a rate higher than        perceptible by the human eye (such as 100 Hz). For example using        90% duty cycle at 100 Hz when a first amount of energy is        available and using 50% duty cycle at 100 Hz when a second        amount of energy is available.    -   Changing the pulse shape of the current being driven through the        load 140 such that the peak current remains relatively constant        while the average current is reduced or increased    -   Reducing or increasing the pulse rate of current through a light        such as an LED at a rate that is perceptible to the human eye.        In a traffic light application this may for example be changing        from 0.5 s on/0.5 s off to 0.4 s on/0.6 s off to reduce overall        power consumption. This also may provide a visual indication of        the status of the system.    -   Reducing or increasing the rate at which an audio message is        repeated. For example, a warning message may be output every 10        seconds when a first amount of energy is available and every 20        seconds when a second amount of energy is available.    -   Reducing or increasing the number of lights such as LEDs that        are illuminated by the EMS 110.    -   Reducing or increasing the amount of time lights such as LEDs        are illuminated during the night. For example the lights may be        turned on for four hours after dusk and 2 hours before dawn when        a first amount of energy is available. Alternatively, the lights        may be turned on all night when a second amount of energy is        available.

FIG. 5 depicts characteristic power versus voltage (PV) plots for solarpanels with and without diode bypassing exposed to differing amounts ofshading. Diode bypassing is a technique whereby individual cells orgroups of cells within a solar panel include standard diodes in parallelto provide a path for current when a particular cell or group of cellsis not producing current due to being shaded from the sun.

Plot 500 shows the PV characteristics for a typical diode bypassed solararray with varying numbers of shaded cells. Plot 550 shows the PVcharacteristics for a typical non-bypassed solar array with varyingnumbers of shaded cells. The individual plots are enumerated as follows:

-   -   505—Diode bypassed solar array with no shaded cells.    -   510—Diode bypassed solar array with one shaded cell.    -   515—Diode bypassed solar array with two shaded cells.    -   520—Diode bypassed solar array with three shaded cells.    -   525—Diode bypassed solar array with four shaded cells.    -   530—Diode bypassed solar array with five shaded cells.    -   535—Diode bypassed solar array with six shaded cells.    -   540—Diode bypassed solar array with seven shaded cells.    -   555—Non-diode bypassed solar array with no shaded cells.    -   560—Non-diode bypassed solar array with one shaded cell.    -   565—Non-diode bypassed solar array with two shaded cells.

In simple solar powered systems with battery charging, the solar panelis sized such that at its maximum power point, its output voltage isslightly higher than the desired voltage to charge the battery. Thebattery will cause the solar panel to operate at a power point slightlybelow its maximum power point and the system will not extract themaximum power from the solar panel. Adding a buck DC/DC converter allowsthe solar panel to be operated at maximum power point regardless of thebattery voltage. But, when solar array cells are shaded, the maximumpower point may drop to a voltage below that of the battery andtherefore, the buck converter will no longer be able to extract maximumpower. For instance, the buck converter may extract maximum power fromthe solar array when the solar array is operating at its maximum powerpoint with no shaded cells (point 502 or 557 on plots 500 and 550). But,a buck converter may not be able to extract maximum power when the solararray is operating with shaded cells (for example point 503 or 562 onplots 500 and 550). The DC/DC converter 201 with its ability to buck,boost or buck/boost allows the EMS 110 to extract maximum power whensome of the cells of solar panel 120 are shaded.

Time

In some applications, it may be desirable for the processor 200 to havean awareness of time of day. Such an application may for example be whenthe load 235 or portions thereof are to be operated at certain times ofthe day. This may include applications where there are more than onesolar powered system within sight of one another. In such a situation,it may be desirable for certain actions such as the activation orextinguishing of the load 140 to happen at the same time. Since thesolar power system 100 may be operated in a location where access tocommunications architecture to retrieve the time of day may beunavailable or impractical, an alternate method of determining time ofday is desirable.

It can be shown that:

MN=(N−k1)*k2 where:

-   -   MN is the number of minutes from sundown to midnight,    -   N is the length of the night in minutes,    -   k1 and k2 are best fit constants derived from historical sunset        and sunrise times for the location where the solar power system        100 is to be installed.

Alternatively, the processor 200 may determine the time of day bymeasuring the time of sunset and sunrise and assuming that a particulartime of day falls half way between these two times. The particular timeof day may be 12:30 am for a location subject to daylight savings timein order that over the entire year, the error in the estimated time ofday is minimized. The particular time of day may be 12:00 am for alocation not subject to daylight savings time. The particular time ofday may also be adjusted depending on the latitude of the solar powersystem 100 within the time zone.

Referring now to FIG. 2 and FIG. 6 the processor 200 may determine theapproximate time of day. The processor 200 open circuits the solar panel120 in order to make an open circuit voltage measurement of the solarpanel. The processor then reads the open circuit voltage using an A/Dinput through conditioning circuitry 210. This open circuit voltagemeasurement is then used in the day/night determination procedure 600.In the following discussion, it will be appreciated that as an alternateto the open circuit voltage, the short circuit current of the solarpanel 120 may also be used as an indication of irradiance.

Starting in the state “day” 605, the processor 200 determines when theopen circuit voltage falls below a first threshold “A”. It then entersthe state “trans1” 610. When in state “trans1”, if the open circuitvoltage rises above threshold “A”, the processor 200 transitions back tostate “day” 605. When in state “trans1” 610, if the open circuit voltageis stable or less than a second threshold “C”, the processor 200 recordsthe present time as the start of night 620, changes the load 235 tonight mode 625 and resets a timer for detection of night end 630. Theprocessor then enters state “night” 635. A stable open circuit voltagemay be determined by the processor 200 calculating that the rate ofchange of the open circuit voltage is below a particular threshold.

While in state “night” 635 if the processor detects that the opencircuit voltage is increasing and is greater than a third threshold “B”,it changes the load 235 to day mode 640 and starts a countdown timer645. The processor then enters state “trans2” 650. While in this state,if the open circuit voltage rises above the first threshold “A”, theprocessor 200 records the present time as the start of day 660 andenters state “day” 605. While in state “trans2” 650 if the timer expiresand the open circuit voltage is still less than the first threshold “A”,the processor changes the load to night mode and enters state “trans3”670. While in state “trans3” if the open circuit voltage rises abovethreshold “A”, the processor changes the load to day mode 680 andfollows the path through block 660. Otherwise, if the open circuitvoltage falls below the third threshold “C”, the processor follows thepath through block 630. This may be due to ambient light causing thesolar panel voltage to increase rather than a true dawn.

In the preceeding description, for an example 12V solar panel, thethresholds may be as follows:

-   -   “A”−11V    -   “B”−6V    -   “C”−4.5V

It will be appreciated that these thresholds may be dynamically ormanually adjusted for a given installation location, solar panel 120type, etc. For example, a given installation may have a higher ambientlight level than another. The processor 200 may thus adjust thethresholds after observation of one or more nights using for exampleaverage open circuit solar panel 120 voltage.

As alternatives or adjustments to the preceeding discussion, othertechniques may be used to determine the beginning of day or night. Forexample, to distinguish between the true start of day and an ambientlight such as a street light turning on, the rate of change of the opencircuit voltage of solar panel 120 may be used. The rate of change ofopen circuit voltage may be much larger when a street light turns onthan the rate of change of open circuit voltage for true dawn.

In the proceeding description, a load in night mode may be for examplean LED at reduced brightness over day mode in a crosswalk, schoolzone or24 hour roadway flasher application to save power due to the fact thatless light is needed during the night for equivalent perception by adriver. In a transit shelter, night mode may be the mode where an LED isilluminated, there being no need to illuminate the shelter during theday.

FIG. 7 depicts an alternate solar powered system 700. This alternatesolar powered system 700 comprises an alternate EMS 710 which is capableof similar functions as the EMS 110, but applies and extracts energyfrom an electrical grid 750 instead of a battery 150. Applying andextracting energy from the electrical grid 750 allows excess energyavailable from the solar panel 120 to be fed to the electrical grid 750(for instance during the summer). It also allows for extraction ofenergy from the electrical grid 750 during times when adequate solarenergy is not available from the solar panel 120 (for instance duringthe winter and/or during the night). In this alternate solar poweredsystem 700, the EMS may comprise an energy meter to measure the amountof energy received from and delivered to the electrical grid 750. Theoutput of this energy meter may be used to bill for the energy usedand/or generated.

It will be appreciated by those skilled in the art that the preferredand alternative embodiments have been described in some detail but thatcertain modifications may be practiced without departing from theprinciples of the invention.

1. An apparatus for powering a load from solar energy comprising: aDC/DC converter; battery terminals coupled to an output of said DC/DCconverter and coupleable to an energy storage device; solar panelterminals coupled to an input of said DC/DC converter and coupleable toa solar panel; logic coupled to said DC/DC converter for controlling aconversion rate of said converter; a detector coupled to said DC/DCconverter for detecting a power output of said converter; wherein saidlogic is operable to adjust the conversion rate until the solar paneloperates at the smaller of: (a) a maximum power point of the solarpanel; and (b) a power point of the solar panel that results in amaximum desired power.
 2. The apparatus of claim 1 wherein: said loadcomprises at least one light emitting diode; and said logic is furtheradapted to control the brightness of said light emitting diode.
 3. Theapparatus of claim 1 wherein said DC/DC converter comprises a buckconverter.
 4. The apparatus of claim 1 wherein said DC/DC convertercomprises a boost converter.
 5. The apparatus of claim 1 wherein saidDC/DC converter comprises a buck/boost converter.
 6. The apparatus ofclaim 5 wherein said DC/DC converter comprises an H-bridge.
 7. Theapparatus of claim 1 wherein said maximum desired power is a function ofan allowable current flow through a component of said apparatus.
 8. Theapparatus of claim 1 wherein said maximum desired power is a function ofa maximum capacity of said DC/DC converter.
 9. The apparatus of claim 1wherein said energy storage device comprises a battery.
 10. Theapparatus of claim 1 wherein said logic is operable to detect that nosolar insolation has been available from said solar panel for anextended period of time and wherein said logic is further operable todisable said DC/DC converter and enter a low power consumption mode uponsaid detection.
 11. The apparatus of claim 10 wherein said detection isindicative of said apparatus being in a shipping container.
 12. Anapparatus for powering a load from solar energy comprising: a DC/DCconverter; battery terminals coupled to said DC/DC converter andcoupleable to a battery; solar panel terminals coupled to said DC/DCconverter and coupleable to a solar panel; first logic for controlling aconversion rate of said converter to operate said DC/DC converter nearthe maximum power point of said solar panel; a detector for detecting anenergy output of said converter; second logic coupled with said detectorand operable to adjust the energy consumption of said load over a periodof time as a function of the energy available from said solar panel oversaid period of time.
 13. The apparatus of claim 12, wherein said secondlogic is operable to adjust the energy consumption of said load to begreater than 85% of the energy available from said solar panel over saidperiod of time.
 14. The apparatus of claim 12 wherein: said loadcomprises at least one light emitting diode; and said logic is furtheradapted to control the brightness of said light emitting diode to adjustthe energy consumption of said load.
 15. The apparatus of claim 12wherein: said load comprises at least one light emitting diode; and saidlogic is further adapted to control a pulse rate of said light emittingdiode to adjust the energy consumption of said load.
 16. The apparatusof claim 12 wherein said DC/DC converter comprises a buck converter. 17.The apparatus of claim 12 wherein said DC/DC converter comprises a boostconverter.
 18. The apparatus of claim 12 wherein said DC/DC convertercomprises a buck/boost converter.
 19. The apparatus of claim 18 whereinsaid DC/DC converter comprises an H-bridge.
 20. A method of determininga time to deliver an amount of energy to a load in a solar poweredsystem comprising: (a) monitoring an output of a solar panel todetermine a start of day; (b) monitoring the output of the solar panelto determine a start of night; (c) rejecting an effect in saiddetermination of said start of day caused by artificial light; (d) usingsaid beginning of said night and said beginning of said day to computethe current time of day; and (e) delivering said amount of energy tosaid load based on said current time of day.
 21. The method of claim 20,further comprising: (f) determining whether the output of the solarpanel has fallen below a first threshold; (g) determining whether theoutput of the solar panel has fallen below a second threshold; (h)determining whether the output has stabilized; (i) determining that (f)and at least one of (g) and (h) have occurred before recording saidstart of night.
 22. The method of claim 21, further comprising: (j)determining whether the output is above a third threshold; (k)determining whether the output has begun to rise; (l) determining thatboth (j) and (k) have occurred before changing said amount of energy toa desired amount of energy for daytime; and (m) determining that theoutput has risen above said first threshold before recording said startof day;
 23. The method of claim 22, wherein said first threshold isgreater than said third threshold and said third threshold is greaterthan said second threshold.
 24. The method of claim 20, wherein saidoutput comprises an open circuit voltage.
 25. The method of claim 20,wherein said output comprises a short circuit current.
 26. The method ofclaim 20, wherein (a) comprises waiting for the rate of change of theoutput to be positive, above a first amount and below a second amount.27. The method of claim 26, wherein the first amount is indicative ofthe rate of increase of light at dawn and the second amount isindicative of the rate of increase of light due to a streetlight. 28.The method of claim 20, wherein said delivering comprises modifying saidamount of energy based on the length of at least one of said day andsaid night.
 29. The method of claim 20, further comprising: (f)monitoring an energy amount generated by said solar panel; and wherein(j) further comprises delivering said amount of energy as a function ofan amount of energy generated by said solar panel.
 30. The method ofclaim 21 wherein (h) comprises determining that the output is indicativeof an amount of light due to a streetlight.